Current efforts are directed towards understanding the relationship between cell cycle regulation and differentiation using the Drosophila compound eye as a model system. Analysis of roughex (rux), a mutation previously shown to be necessary for G1 arrest during eye development, demonstrated that rux functions by regulating the stability of the G2 cyclin, Cyclin A (CycA). In recent experiments, we showed that Rux can bind directly to CycA via a motif, RXL, which has been shown to be necessary for cyclin binding to cyclins in mammalian cyclin kinase inhibitors and other cyclin-binding proteins. A bipartite nuclear localization signal (NLS) was also identified; deletion of this domain resulted in cytoplasmic accumulation of CycA. Cytoplasmic CycA was not competent for mitosis and S phase was prolonged in cells expressing the Rux NLS mutant, suggesting that CycA must translocate to the nucleus for its S/G2 functions. These findings are currently in press. The rux mutation was used in a genetic screen to identify other loci involved in the developmental control of cell cycle progression. We focused on a single suppresser, S42/S56, for which there also existed a P-element allele. Cloning of the gene identified a very large protein with similarity to the human fragile X mental retardation 2 gene and the AF4 protooncogene. Phenotypic analysis showed no cell cycle phenotype and a strong maternal effect. This gene was also isolated in a number of other genetic screens, and was subsequently determined to be a transcription factor. Current efforts are directed towards the cloning and characterization of an eye-specific suppresser of rux that may be involved in down-regulation of Rux protein via CycE-mediated proteolysys, and the cloning of a mitotic regulator that also participates in regulating cell cycle progression during Drosophila eye development.